When Apple released macOS High Sierra 10.13.1 on Tuesday it also rolled out security updates for macOS El Capitan and macOS Sierra to patch the KRACK Wi-Fi security flaw.
KRACK is a vulnerability in the WPA2 encryption standard for Wi-Fi networks that lets attackers decrypt the data passing to and from a specific device. The vulnerability exists in most any device capable of using WPA2 on Wi-Fi networks and once it was publicly disclosed product manufacturers started scrambling to release patches.
The security update should show up in the Mac App Store Updates section. You can also download the El Capitan and Sierra update installers from Apple’s website.